Deprojecting beam systematics for next-generation CMB B-mode searches

Understand

Measurements of the cosmic microwave background polarization are vulnerable to systematic contamination from beam imperfections.

• Because the unpolarized CMB T is orders of magnitude larger than the polarized E and B signals, even a tiny difference in instrument response between two orthogonally polarized measurements of the CMB will result in a large non-zero differential signal, even if the CMB is unpolarized.
• Two strategies to mitigate this temperature-to-polarization leakage are the use of a rotating half-wave-plate and the fitting and removal of leakage templates from the polarized signal.
• The half-wave-plate approach will, in principle, work for arbitrary beam shapes, but in practice introduces complicated additional optics that themselves can introduce systematics.